1. Field of the Invention
The present invention relates to a fault current limiter and in particular a fault current limiter having a re-settable static magnetomotive force source.
2. State of the Art
A Fault Current Limiter (FCL) is a device used to limit, or in its most basic form interrupt, a fault current in a branch of a circuit on occurrence of a fault condition so as to prevent any components in the circuit from being overloaded.
Fuses are an example of a device which interrupts high currents in fault conditions, however these devices must be replaced after a fault condition has occurred and cannot be used in high power systems. It is usually more preferable to employ a re-settable FCL which limits the fault current, rather than interrupts it.
An example of a re-settable FCL which is suitable for low power operations is the Magnetic Current Limiter (MCL) which comprises a permanent magnet sandwiched between a saturable core with an AC wire wound around the core (see FIG. 1). The permanent magnet 2 causes the core to saturate in the normal operating state. For the device to operate for each half of the AC cycle, two cores are required such that in the first core the magnetic field produced by the AC current flows through the coil since the magnetic field provided by the permanent magnet are additive and in the second core they are subtractive. In the normal operating condition the AC current flowing through the coil is low and both cores are saturated causing the effective impedance in the AC coil 3 to be low. During a fault condition a large AC current value (the fault current) forces each of the cores of the device to come out of saturation in alternative half-cycles. The mostly unsaturated first core in combination with the mostly saturated second core (and vice versa) restricts the flow of the fault current since the inductance of the coil is caused to increase. In this arrangement multiple distinct core elements are used and useful regions of the core, where interaction between the magnetic field associated with the magnet and the AC coil takes place, are limited. Further, the MCL does not perform well in high power alternating systems.
In higher power alternating power systems, series reactors have been implemented so as to protect against excessively large currents under short-circuit, however they have a major disadvantage in that they produce significant I2R losses.
Another system suitable for use in a high power alternating system is the saturated Iron Core FCL which comprises a copper coil with an iron core. The Iron core is maintained in magnetic saturation in normal operation by applying the magnetic field of an additional superconducting wire. The impedance of the device is low in normal operation; however when a fault condition occurs the increased AC current through the normal conducting coil causes the core to depart from saturation so as to cause the impedance of the device to increase. In this arrangement the superconducting wire is exposed only to DC current and therefore always remains in the superconducting state and eliminates the need for a recovery time. The main disadvantages with this system includes the large mass and volume of the device, the high magnetic fields at the superconducting coil and the high cooling costs of the superconducting coil.
A recently developed system for use in high power applications is the Superconducting FCL which relies on a rapid change of resistance with temperature so as to limit the fault current. The superconducting FCL is directly connected in series with the current path to be protected. When a specified current density is reached, which corresponds to a particular temperature, the resistance increases rapidly so as to substantially limit the flow of the fault current. Such arrangements have an array of disadvantages including: a) expensive cooling mechanisms since the superconductor must be cooled to 77K, b) the development of thermal instabilities and c) AC current cooling losses. Further, in order to prevent the excessive heating of the superconductor, so as to avoid long cool down phases, the reduced fault current is only be carried for a few cycles.
WO200702924 discloses a fault current limiter device in which an electromagnet DC source is utilised.
Due to the costs associated with the Saturated Iron Core FCL and the Superconducting FCL such systems are not usually desirable for smaller power operations e.g. circuits implementing power electronics devices such as transistors, diodes etc.